The electrophoretic display is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a solvent. This general type of display was first proposed in 1969. An electrophoretic display typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. One of the electrodes is typically transparent. A dispersion composed of a colored solvent and suspended charged pigment particles is enclosed between the two plates.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side by attraction to the plate of polarity opposite that of the pigment particles. Thus the color showing at the transparent plate may be determined by selectively charging the plates to be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of grey) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages.
There are several types of electrophoretic displays available in the art, for example, the partition-type electrophoretic display (see M. A Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol ED 26, No. 8, pp 1148-1152 (1979)) and the microencapsulated electrophoretic display (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026). In a partition-type electrophorectic display, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of the particles such as sedimentation. The microencapsulated electrophoretic display has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent.
Furthermore, an improved electrophoretic display (EPD) technology was recently disclosed in the co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 now pending, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 now U.S. Pat. No. 6,672,921 and U.S. Ser. No. 09/784,972, filed on Feb. 25, 2001 now pending, all of which are incorporated herein by reference. The improved electrophoretic display comprises cells formed from microcups of well-defined shape, size, and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. An improved liquid crystal display (LCD) technology was disclosed in the co-pending application, U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001 now U.S. Pat. No. 6,995,138, the content of which is incorporated herein by reference.
Multifunctional UV curable compositions have been employed to fabricate the microcup array for the improved electrophoretic display. However, the microcup structure formed tends to be quite brittle. The internal stress in the microcups due to the high degree of crosslinking and shrinkage often results in undesirable cracking and delamination of the microcups from the conductor substrate during demolding. The microcup array prepared from the multifunctional UV curable compositions also showed a poor flexure resistance.
It has now been found that resistance toward flexure or stress may be significantly reduced if a rubber component is incorporated into the microcup composition. Two other key properties: demoldability during microembossing and adhesion between the sealing layer and the microcups have also been considerably improved with the composition containing this additional rubber component.
Suitable rubber materials for this purpose include SBR (styrene-butadiene rubber), PBR (polybutadiene rubber), NBR (acrylonitrile-butadiene rubber), SBS (styrene-butadiene-styrene block copolymer), SIS (styrene-isoprene-styrene block copolymer), and their derivatives. Particularly useful are functionalized rubbers such as polybutadiene dimethacrylate (CN301 and CN302 from Sartomer, Ricacryl 3100 from Ricon Resins Inc.), graft (meth)acrylated hydrocarbon polymer (Ricacryl 3500 and Ricacryl 3801 from Ricon Resins, Inc.), and methacrylate terminated butadiene-acrylonitrile copolymers (Hycar VTBNX 1300xc3x9733, 1300xc3x9743 from BFGoodrich).